37 lines
982 B
Scheme
37 lines
982 B
Scheme
;;; NQUEENS -- Compute number of solutions to 8-queens problem.
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;; 2006/08 -- renamed `try' to `try-it' to avoid Bigloo collision (mflatt)
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(define trace? #f)
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(define (nqueens n)
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(define (one-to n)
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(let loop ((i n) (l '()))
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(if (= i 0) l (loop (- i 1) (cons i l)))))
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(define (try-it x y z)
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(if (null? x)
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(if (null? y)
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(begin (if trace? (begin (write z) (newline))) 1)
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0)
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(+ (if (ok? (car x) 1 z)
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(try-it (append (cdr x) y) '() (cons (car x) z))
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0)
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(try-it (cdr x) (cons (car x) y) z))))
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(define (ok? row dist placed)
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(if (null? placed)
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#t
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(and (not (= (car placed) (+ row dist)))
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(not (= (car placed) (- row dist)))
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(ok? row (+ dist 1) (cdr placed)))))
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(try-it (one-to n) '() '()))
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(let ((input (with-input-from-file "input.txt" read)))
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(time
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(let loop ((n 500) (v 0))
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(if (zero? n)
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v
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(loop (- n 1) (nqueens (if input 8 0)))))))
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